Project 1: Influence of Apolipoprotein E Structure on Function: Apolipoprotein (apo-)E is a major mediator of lipoprotein metabolism via its interaction with members of the low density lipoprotein (LDL) receptor family. Polymorphic differences in the structure of apo-E have a major impact on apo-E-mediated lipoprotein metabolism. In addition, apo-E4, a structural isoform, is a major risk factor for Alzheimer's disease and other neurodegenerative disorders. Therefore, it is critically important to understand the influence of apo-E structure on its functions. The overall objective of this proposal focuses on the structure/function relationships of apo-E-related to lipoprotein metabolism, using x-ray crystallography as the primary structural tool. Previously, we introduced the concept of domain interaction-that the two structural domains of apo-E influence each other's functional properties-and determined how this interaction directs apo-E4's binding preference for very low density lipoproteins. In this proposal, we will extend our studies on domain interaction to elucidate the increased receptor-binding activity of apo- E4, to explore other aspects of domain interaction in both in vitro and in vivo model systems, and to gain a better understanding of the structure of lipid-bound apo-E. In addition, we will determine the structure of the LDL receptor and define the structure of the LDL receptor and define the molecular interface of the receptor and bound apo-E. We propose two specific aims to achieve these objectives. In Aim 1, we will determine the x-ray structures of the LDL receptor and several physiologically relevant forms of apo-E. In addition to lipid-free forms, the structures of lipid- associated or detergent-treated apo-E will be determined to define the structural change that occurs when apo-E goes from a lipid-free, receptor- inactive state to a lipid-associated, receptor-active state. In Aim 2, we will study the effect of domain interaction on receptor-binding function and test the hypothesis that a second distinct group of arginine residues within the hinge region of apo-E contributes to receptor-binding activity. The proposed studies will lead to new insights into how the structural differences in apo-E contribute to isoform-specific effects in lipoprotein metabolism, heart disease, and Alzheimer's disease.